The present invention is related to a plunger stroke pipette for replaceable pipette points.
Plunger stroke pipettes serve for dosing liquids. They take up a certain volume of a sample liquid in a pipette point. As a disposable article, the pipette point is normally realised from plastic material. After one dosing, the pipette point can be replaced through a new pipette point. Thus, contamination of the plunger stroke pipette and carry-over of sample liquid is avoided.
For the aspiration of sample liquid into and the ejection of sample liquid from the pipette point, the plunger stroke pipette has a cylinder with a plunger, which is shiftable therein. Via a connection channel, the cylinder is connected with openings in a seat for detachable mounting of a pipette point. The plunger is coupled to a driving equipment, which serves for the displacement of the plunger in the cylinder. The plunger is shiftable in the cylinder between an upper and a lower stopper. Upon displacement, the plunger moves an air column, through which liquid is aspirated into the pipette point or ejected from it, respectively. In the beginning of the aspiration of air into the cylinder, the counter-stopper is situated on the lower stopper. In the beginning of the displacement of air from the cylinder, the counter-stopper fits closely to the upper stopper. The amount of liquid which is taken up or discharged, respectively, depends of the stroke of the plunger.
The stroke volume of the plunger does not exactly correspond to the amount of liquid which is taken up and discharged. As the air column lengthens somewhat through the weight of the liquid, the stroke volume exceeds the liquid volume. In particular, the difference between stroke volume and liquid volume depends from the density and the viscosity of the liquid, the temperature, the air pressure and from wetting effects. It is known to adjust the position of the upper stopper for the calibration of plunger stroke pipettes to a certain dosing volume.
In fixed volume pipettes, the distance between upper and lower stopper is constant. A fixed volume pipette with an upper stopper in the form of a threaded sleeve which is adjustable by means of a calibration tool is known from U.S. Pat. No. 4,020,698, the entire contents of which is hereby incorporated by reference in its entirety. The sleeve-like calibration part has a scale on its outer side, which is visible from the outside through a transparent reading window.
In plunger stroke pipettes with adjustable dosing volume, the position of the upper stopper is changeable. Adjustable plunger stroke pipettes have an adjustable spindle for changing the position of the upper stopper. The difference of the stroke volume from the dosing volume can be different in the adjustment range of dosable liquid volumina. Plunger stroke pipettes are known in which the indicating equipments can be uncoupled from the adjustment equipments for the upper stopper for the purpose of calibration. DE 43 35 863 C1 (the entire contents of which is hereby incorporated by reference in its entirety) discloses such a plunger stroke pipette with uncouplable gearwheels between indicating equipments and adjustment equipments. In order to uncouple, a control lever which projects to the outside through an opening of the pipette casing has to be actuated.
In the known plunger stroke pipettes, a manufacturer's calibration is performed by the pipette manufacturer under standard conditions. In this, bi-distilled water of a temperature of 20 to 25° C. and an air pressure of 1013 mbar is used. If the user wants to pipette at conditions different from these, (an other liquid than bi-distilled water, e.g.), he has to change the manufacturer's calibration. The change of the manufacturer's calibration mostly necessitates sumptuous use of tools. Finding the manufacturer's calibration again necessitates sumptuous gravimetric measurements.
Departing from this, the present invention is based on the objective to provide a plunger stroke pipette which facilitates change of manufacturer's calibration and finding manufacturer's calibration again for the user.
Advantageous embodiments of the plunger stroke pipette are indicated in the subclaims.